1. Field of the Invention
The present invention relates to a photoelectric smoke detector and more particularly to the structure of the light-sensing system within its smoke detecting unit.
2. Description of the Prior Art
In the prior art, formed inside a photoelectric smoke detector of the scattered light type is a smoke detecting chamber which communicates with the exterior in a condition where the detector is attached to a ceiling so that in a monitoring mode infrared light is intermittently irradiated into the smoke detecting chamber and a light-sensing element detects the scattered light of the infrared light caused by smoke entering into the smoke detecting chamber. The smoke detecting chamber is generally in the form of a flat cylindrical space which extends along the ceiling and whose outer periphery is surrounded by an insect screening, and arranged on the inner side of the insect screening are a plurality of labyrinth plates which serve the purpose of ensuring the communication with the exterior and preventing the entry of light from the exterior.
A light source including an infrared light emitting diode is arranged at the position of some labyrinth plates within the smoke detecting chamber and the infrared light emitting diode intermittently irradiates infrared light within a relatively wide range of angles in the width direction inside the chamber. Arranged at another position between other labyrinth plates within the smoke detecting chamber is the light-sensing element whose optical axis is extended in a direction which intersects the optical axis of the infrared light emitting diode, and disposed between the light source and the light-sensing element is a light shielding member for preventing the irradiated light from the infrared light emitting diode from directly falling on the light-sensing element. Also arranged at still another position near the peripheral wall of the smoke detecting chamber is a test light emitting diode for irradiating a test light and it faces the light-sensing element. The light-sensing element is generally composed of a photodiode covered with a shielding cap and it has a light sensitivity to both the infrared light from the infrared light emitting diode and the visible light from the test light emitting diode.
When smoke enters into the smoke detecting chamber from the outside through between the insect screening and the labyrinth plates, the infrared light irradiated from the light source is scattered by smoke particles and a part of the scattered light falls on the light-sensing element. The light-sensing element generates a detection output synchronized with the intermittent emission of light from the infrared light emitting diode or the light source so that its magnitude is detected in terms of a smoke density by a suitable electric signal processing circuit and it is utilized for the determination of a fire.
With the above-described conventional photoelectric smoke detector having the thin-type flat smoke detecting chamber, if it is desired to improve the sensitivity of the detector, there is a limitation to the optical detection characteristic of the smoke detecting unit itself and generally it is necessary to increase the gain of the signal amplifying system of the electric signal processing circuit within the detector. In this case, however, the gain in the noise component of the detection output from the optical system is also increased with the result that the detection output is varied considerably by a slight change in the characteristics, such as, the effect of the optical external noise within the smoke detecting chamber and the contamination of the chamber inner wall and the optical system.